Our aim is to study the organization of neurotransmitter receptors on nerve and muscle cells in relationship to the development and function of synapses. Our recent work has focused upon the factors, extinsic and intrinsic to the developing skeletal muscle fiber, which regulate the distribution of nicotinic acetylcholine receptors. Acetylcholine receptor aggregation is induced on cultured myotubes by neuronal factors, and this system is used to study the mechanisms of receptor aggregation, as well as the stabilization or elimination of aggregates which occur in developing neuromuscular junctions. Our major findings in the past year are as follows: 1) Newly-formed receptor aggregates are reorganized in the continued presence of embryonic brain extract (receptor aggregation factor) to form a smaller number of larger aggregates. These aggregates are twice as resistant to disassembly by temperature elevation to 38 degrees or exposure to sodium azide. This maturation of receptor aggregates is analogous to changes which occur during neuromuscular junction formation. 2) Immunofluorescence staining shows that newly-formed receptor aggregates are associated with concentrations of the cytoskeletal proteins vinculin and Alpha-actinin, which have been implicated in membrane-actin interactions. 3) Crude as well as highly-purified factors from the extracellular matrix of Torpedo electric organ induce rapid formation of receptor aggregates in our system, but the aggregation pattern differs from that induced by embryonic brain extract.